My sincere thanks for the effort.

You're welcome!

for you in a very similar configuration. My MacBook is 2.9 GHz vice your
Air looks to be 1.7 GHz - there wouldn't be any clock-counting timeouts in this
software, would there?.

2.0Ghz i7 here; but no clock speed should have zero impact on performance of the USB cable. There are separate clocking for each sub-system these days as well.

Is there anything reasonable I can do to troubleshoot this?

Reading the later emails, I think your best bet is to do what you've done and order a new cable... I'd recommend sticking with that driver first and see how it works. I have 4 different USB cables (and radios) all told and that driver worked for all of them. Hopefully you just had a bad cable.

When Chirp decides it's done after half a second, are there any logs
to look at, or any way to get visibility into what it thinks it's doing?

There is a debug.log... it is at: ~/.chirp/debug.log ... that said, I don't think it'll help, but might want to check the end of it and see what you see.

I'd suspect the cable, but it works for read, and I'm reluctant to
spend the money and wait time to get another without some confidence
that that's the problem, and it isn't feeling that way to me so far.

I think trying a new cable is definitely the right next step. I hope it works for you!

Jay